Increasing worldwide concern over aircraft noise during landing and takeoff, and its impact on communities surrounding commercial airports, has led to more stringent regulation of permissible engine noise. Proposed regulations would restrict many older second-generation aircraft from landing in some of the world's airports because of excessive noise levels.
In the United States, the National Aeronautics and Space Administration (NASA) has proposed a goal of reducing jet noise by 7-10 EPNdB by the year 2000, and the International Civil Aviation Organization (ICAO) is considering imposing stricter noise standards internationally. Such goals and regulations could sideline many aircraft that would otherwise have decades of useful service left. See, e.g., Published Goals, NASA Advanced Subsonic Technology Program.
The aircraft industry currently relies on passive noise suppression hardware to reduce engine noise, which consist of arrays of resonators that line the interior surface of engine ducts. These are typically constructed with porous air-passage face sheets that are bonded to closed back cavities. The cavities commonly comprise honeycomb cells that are bonded to a solid backing plate that provides a rigid reflecting surface. These nacelle liners, however, are limited at reducing engine noise to meet potential stricter noise standards. This problem is exacerbated by trends in new jet-engine designs to have shorter engine ducts with larger diameters.
Properly configured multi-layered passive liners increase the sound attenuation of turbofan engines, but increase size, cost and weight compared to single degree-of-freedom liners. Thus these liners may not be feasible for meeting future international noise regulations.
To date, one approach to solve this problem is application of passive hush kits that provide only modest noise reduction while increasing fuel consumption and decreasing engine performance. These hush kits are merely stop-gap measures, barely bringing engines into compliance with current noise regulations. Consequently, there is a need for a fundamentally different acoustic system that can be retrofitted into existing turbofan engines to bring thousands of current turbofan engines into compliance with future noise abatement requirements without decreasing performance. In addition, it would be highly desirable if such a system was relatively inexpensive to manufacture, install and maintain.
Recently, acoustical researchers have turned to active noise control to attenuate undesirable noise by measuring the frequency, phase and amplitude of the noise using microphones and signal-processing techniques to generate anti-waves to cancel or reflect the noise. This technique, however, has been difficult to apply to real-world phenomena with complex sound-field patterns, particularly to cancel the highly complex radial and spinning modes generated by the rotor-stator interactions within jet aircraft engines.
One promising line of active noise control research employs sound absorption, rather than cancellation or reflection of the noise, by generating acoustic waves within Helmholtz resonators, thereby optimizing the resistance and reactance of the resonator. Installing active-control transducers within resonator cavities protects the transducers from the harsh environment within turbofan engines, increasing their efficiency over a broad range of frequencies above and below the naturally tuned frequency of the resonator. The use of active control to re-tune a resonator is described in detail in Hersh et al., U.S. Pat. No. 5,119,427, which is incorporated by reference herein.
All active noise control techniques face numerous technical hurdles before they can effectively absorb, cancel or scatter sound in complex sound-field patterns such as those generated by the rotor-stator interaction in turbofan engines. One such obstacle is the amount of acoustic energy that is required to absorb or reflect the high-amplitude noise within a turbofan engine. But the primary obstacle that remains to be solved is to determine and create proper sound wave patterns to absorb, cancel or scatter the complex radial and spinning modes within turbofan engines.
The rotor-stator interaction in a turbofan engine generates spinning and radial modes in which each sound mode propagates out of the inlet or exhaust duct at an angle relative to the duct axis. This propagation angle is dependent upon the mode structure and the sound frequency. Typically, the modes with simpler structure propagate with smaller angles relative to the duct axis than the more complex or higher-order modes. This results in relatively few encounters by the wave front with the acoustic liner on the periphery of the inlet or exhaust duct, limiting the effectiveness of the acoustic liner to absorb noise. This problem is exacerbated by modern turbofan engines that have very short inlet and exhaust ducts.
Experiments were conducted in the 1970's using segmented passive liners in which a first passive liner segment modified the boundary condition along the duct wall, which changed the average propagation angle of the sound field so that it encountered the passive liner of the second and other segments at greater angles and thus with more encounters. This resulted in greater attenuation of the segmented system than a uniform passive liner without the mode-scattering effect of the first segment. See Sawdy, D. T., Beckemeyer, R. J. and Patterson, J. K., "Analytical and Experimental Studies of an Optimum Multiple Segment Phased Liner Noise Suppression Concept," NASA CR-134960, May 1976; Lester, H. C. and Posey, J. W., "Optimal One-Section and Two-Section Circular Sound Absorbing Duct Liner for Plane-Wave and Monopole Sources without Flow," NASA TN D-8348, December 1976; and Kraft, R. E. and Paas, J. E., "Effects of Multi-Element Acoustic Treatment on Compression Inlet Noise," AIAA Paper No. 76-515, Presented at 3.sup.rd AIAA Aeroacoustics Conference, Palo Alto, Calif., July 1976. Research has recently shown that the propagation angles were intimately related to the mode cut-off ratios, attenuation in the duct liners, and the far-field radiation angles. Rice, E. J., "Modal Propagation Angles in Ducts with Soft Walls and Their Connection with Suppressor Performance," NASA Technical Memorandum 79081, Presented at 5th AIAA Aeroacoustics Conference, Seattle, Wash., March 1979.
Another way of understanding the propagation angle relative to frequency is in terms of the mode-cutoff ratio. Each mode in a cylindrical or annular duct is characterized by a circumferential and radial structure that may be defined in terms of the circumferential periods (the number of times the wave repeats around the circumference of the duct) and the number of radial pressure nodes. Typically, these are identified by the indices m and n, respectively, using nomenclature (m,n). Thus, mode (0,0) is a simple axial wave with no structure and propagates parallel to the duct axis. Mode (1,0) has circumferential structure that repeats once around the duct and has no radial nodes. Mode (13,2) repeats 13 times around the duct and has two radial nodes. Each of these modes is characterized by a lower limit frequency that is inversely proportional to the duct diameter, such that at frequencies less than the limit, the duct does not support wave motion in that mode. This limit is defined as the modal cutoff frequency. The cutoff ratio is the ratio of the sound frequency to the cutoff frequency for a given mode and duct size.
The propagation angle is a monotonically decreasing function of the cutoff ratio. For a cutoff ratio of unity, the propagation angle is 90 degrees (perpendicular to the duct axis); for extremely large cutoff ratios, the propagation angle approaches 0 degrees (parallel to the duct axis). The actual expressions for cutoff frequency as functions of m and n are determined from solutions to Bessel's equation and are not simple expressions. However, a good rule of thumb is that higher mode indices result in lower cutoff ratios and therefore higher propagation angles.
Most experiments on passive segmented liners conducted in the 1970s involved rectangular ducts with two-dimensional sound fields generated by loudspeakers to verify the general theory. Some experiments investigated circular ducts with two-dimensional sound fields. Most of these experiments showed reasonable agreement with the theory after employing iterative adjustments to the initial modal structure and the wall impedance.
To our knowledge, only one experiment tested the performance of a passive segmented liner system to sound fields resembling the spinning modes produced by the rotor-stator interaction of a turbofan engine. See Lester, H. C. and Posey, J. W., "Duct Liner Optimization for Turbomachinery Noise Sources," NASA Technical Memorandum X-72789, November 1975. In the test, a high-speed 12-inch fan was used to create spinning modes and flow. The test, as did the simpler two-dimensional sound-field experiments, showed a large potential increase in acoustic attenuation for intermediate frequency ranges. The optimum first liner segment tended to have a purely reactive impedance with a very low resistance, which caused minimal dissipation of the sound energy.
The conclusion reached from these tests was that the multi-segmented passive liner system improved sound attenuation by scattering the sound field into higher-order radial modes, increasing the average propagation angle of the sound field. Higher-order radial modes with increased average propagation angles relative to the duct axis are more efficiently attenuated by the second passive liner segment.
Despite some encouraging initial results, studies on multi-segmented passive liners ceased in 1979 after Baumeister published a statistical survey that showed multi-segmented passive liners were very sensitive to the relative amplitude and phases of the incident sound fields. Thus their performance was enhanced only very near their optimum design conditions, such as at a particular engine rotational speed. Baumeister, K. J., "Evaluation of Optimized Multi-sectioned Acoustic Liners," AIAA Journal, Vol. 17, No. 11, pp. 1185-1192, November 1979. At other rotational speeds, the performance of the segmented liner was no better than that of conventional uniform impedance liners. His conclusion was that segmented liners were not worth pursuing, and Baumeister and others abandoned research in this area. A review of the literature suggests that there has been no further publication nor substantial progress in this technology since 1979.